“All of these methods are ultimately leading toward having students do more mental math, said Briars. The 2nd grade standards require students to “fluently add and subtract within 20 using mental strategies” (CCSS.MATH.CONTENT.2.OA.B.2). So while the strategies may take longer at first—even three times longer, as Ellis said—eventually under the common-core students should be able to do these kinds of problems in an instant.